1. Field of the Invention
The present invention relates to a signal analysis method and circuit, and more particularly, to a signal analysis method and circuit used in an induction type power supply system.
2. Description of the Prior Art
In an induction type power supply system, each of the power supplying terminal and the power receiving terminal includes a coil for performing induction. When the coils are operated, the relative distance between the two coils is usually smaller than the diameter of the coils. With the small distance between these two coils, the electrical characteristics of the coils may be mutually influenced during the induction process. In the induction type power supply system, sending of power is controlled by the power supplying terminal, and the power output status is monitored by the power receiving terminal, wherein the power receiving terminal needs to transmit data to the power supplying terminal to communicate with the power supplying terminal. Since there is no physical connection media between the power supplying terminal and the power receiving terminal, the communication should be performed via wireless communication technology. A common communication method applied in the industry is that the power receiving terminal varies the electrical characteristics of the receiving-end coil using signal modulation technology, where the variations of the electrical characteristics are reflected to the power supplying terminal to generate signal variations on the supplying-end coil. The power supplying terminal then recovers the modulation signals of the power receiving terminal via demodulation technology, and then decodes a combination of modulation signals to obtain a data code.
However, there are still several problems which cannot be effectively solved by the above method. First of all, the power receiving terminal generates the modulation signals and transmits the modulation signals to the power supplying terminal, and the power supplying terminal then performs demodulation on the received signals. The modulation and demodulation scheme is similar to the amplitude modulation (AM) of the wireless communication technology, where a circuit element such as an envelope detector or a low-pass filter is used to filter out the high frequency component of the signal, in order to form a low frequency signal. This implementation is feasible if two conditions are met. The first one is that the carrier frequency should be far higher than the frequency of modulation signal, and the second one is that the carrier should be relatively stable and the modulation degree should be large enough. The operating frequency in an induction type power supply system is substantially equal to 125 kHz due to the characteristics of the switching elements and the regulations of electromagnetic interference limitation. In such a situation, in order to clearly separate the modulation signal and the carrier signal, the frequency ratio of these two signals should be up to 100 or more. Since the carrier signal is a low frequency signal, the data frequency that can be transmitted by the carrier should be much lower. This limits the speed of data transmission.
In addition, several power supplying terminals are configured to control the amplitude variations of the resonant voltage by varying the driving frequency of the coil, in order to vary the magnitude of output power. Therefore, the carrier frequency of the signals on the supplying-end coil may not be fixed, such that the power receiving terminal is required to perform modulation in different carrier frequencies. However, in the power supplying terminal, the envelope detector used for detecting the modulation signal may not be corresponding to different frequencies, and therefore cannot accurately retrieve the modulation signal. In addition, during operations of the induction type power supply system, the value of the resonant voltage may change with the magnitude of output power. For example, when the power supplying terminal outputs low power, e.g., 1 Watt, a resonant voltage of 20 Volts may be enough. When the output power of the power supplying terminal increases to 100 Watts, the resonant voltage may reach 200 Volts or more. However, a general envelope detector cannot remain satisfactory capability throughout the high range of resonant voltage. In other words, since the envelope detector and filter circuit have a poor dynamic range, the envelope detecting capability in the prior art has a formidable difficulty.
As mentioned above, two conditions should be met to clearly analyze the signals by the conventional signal modulation method used for the induction type power supply system. The first condition is that the modulation degree should be large enough, and the second is that the modulation frequency should be far lower than the carrier frequency. The above carrier is provided by the power supplying terminal, and the modulation degree is determined by the modulation strength of the power receiving terminal, where the modulation strength refers to the strength applied on the receiving-end coil to vary the electrical characteristics of the coil. In general, the signal modulation is to vary the resonant point by varying the impedance or electrical characteristics of the coil or adding a capacitor connected to the receiving coil in parallel, allowing the signal to reflect to the supplying-end coil to vary its signal amplitude. However, the higher modulation degree may affect the power output capability in the back-end more easily since the modulation process may generate a load on the coil. In addition, when the system is operated in a higher power, the power outputted to the back-end load from the receiving-end coil may reach an upper limit. At this moment, it is hard to insert a variation on the electrical characteristics of the receiving-end coil via modulation operations. That is, neither increasing nor decreasing the load on the coil can generate an enough modulation degree. For example, the load varying capability of the modulation circuit may be 1 Watt. When the power receiving terminal outputs power equal to 5 Watts, the variation ratio generated via coil modulation may achieve 1/5, which can be reflected to the supplying-end coil and the modulation degree is still enough. When the power receiving terminal outputs power equal to 100 Watts, the variation ratio generated via coil modulation is only 1/100. Therefore, the modulation method cannot be operated with high output power. In fact, the load varying capability of the modulation circuit possesses an upper limit since increasing of load may burn the circuit and decreasing of load may reduce the power output efficiency in the back end.
On the other hand, the modulation signal belongs to digital data. In the prior art, the modulation should be continuously performed for a specific period of time, where the frequency of the modulation signal should be at least 100-fold lower than the frequency of the carrier, in order to be separated by the filter of the power supplying terminal. In other words, the length of the modulation signal should be longer than or equal to 100 resonant cycles. However, power loss may occur during the modulation process, and the lower modulation frequency decreases the data transmission speed, which reduces the performance of the induction type power supply system. In brief, the modulation strength should be increased in order to increase the modulation degree, and the modulation frequency should be decreased in order to allow the filter to clearly separate the carrier and the modulation signal. The modulation technology in the prior art may not meet these two conditions, and therefore the application of the legacy modulation technology has a formidable difficulty.
Thus, there is a need to provide a new signal analysis method and circuit to obtain a more preferable signal analysis performance and also prevent the above problems.